The subject matter disclosed herein relates generally to balance-unbalance (balun) devices, and more particularly to a quarter-wave balun used in a medical imaging system.
Magnetic Resonance Imaging (MRI) systems include a magnet, such as a superconducting magnet that generates a temporally constant (i.e., uniform and static) primary or main magnetic field. MRI data acquisition is accomplished by exciting magnetic moments within the primary magnetic field using magnetic gradient coils. For example, in order to image a region of interest, the magnetic gradient coils are energized to impose a magnetic gradient to the primary magnetic field. Transmit radio-frequency (RF) coils are then pulsed to create RF magnetic field pulses in a bore of an MRI scanner to selectively excite a volume corresponding to the region of interest in order to acquire MR images of the region of interest using receive RF coils. During the transmission of the RF magnetic field pulses, the receive RF coils are decoupled. The resultant image that is generated shows the structure and function of the region of interest.
There are typically balancing problems that occur when the MRI system is operated at the relatively high frequencies, especially, when coupling balanced and unbalanced circuits, such as transmission lines, together. For example, the RF coils are generally connected to the MR system, and more particularly to an RF generator and/or an RF receiver using a coaxial cable. The coaxial cable facilitates signal transmission and protecting the MRI system from “picking-up” extraneous RF signals which are present in the environment. The conventional coaxial cable includes a braid separated from a central conductor by a plastic material. The braid acts as a shield that minimizes the pick-up of foreign frequencies by the coaxial cable. However, the braid itself may carry foreign induced currents. The induced currents may flow into the RF coil and cause inaccuracies in the measurements being taken from the RF coil.
Therefore, a balun (balance/unbalance) device is used to reduce the noise and/or stray RF currents generated due to induced currents in the co-axial cable. Specifically, the conventional balun creates a high impedance on the RF cable when the overall length of the balun is equal to one quarter of the electromagnetic radiation wavelength of the signal transmitted through the RF cable.